Optical fiber communication systems typically comprise a modulated laser source, a length of optical transmission fiber and a receiver. Light from the laser is typically modulated into signal pulses at a high bit rate which may exceed 10 G bits/s and launched into the transmission fiber. The transmission fiber carries the signal pulses to the receiver where the signal is demodulated. Intermediate between the transmitter and the receiver, the transmission path may include optical fiber amplifiers to compensate amplitude loss, add/drop modes to permit the addition or dropping of signal channels at intermediate points, and chromatic dispersion compensators to compensate the tendency of different wavelength components of pulses to travel through the fiber at slightly different speeds.
In the absence of chromatic dispersion compensation, the different wavelength components in a transmitted pulse will gradually separate with increasing transmission distance, spreading out the pulse in time. The faster spectral components—typically the shorter wavelength components—arrive first and the slower components (usually long wavelength) arrive last. Eventually the components of a pulse will spread sufficiently that the fast components will arrive at the same as the slow components of a preceding pulse. In such case, it becomes difficult to separate pulses, and signal information may be lost. Chromatic dispersion is compensated by including in the transmission path a component, such as a length of dispersion compensating fiber, which slows the fast components and speeds up the slow components.
Another type of dispersion known as polarization mode dispersion (PMD) has only recently been recognized as a problem in contemplated high speed optical fiber systems (≧10 Gbits/s). PMD is pulse dispersion due not to the wavelength components of the transmitted light but rather to the polarization components. The light in a pulse may be thought of as partitionable into two different orthogonal polarization directions. If the pulse is passing through a symmetrical, homogeneous fiber, these two components will travel at the same speed, maintaining the compact shape of the pulse. However if the pulse encounters minor imperfections such as slight deviation of the fiber from circular cross section, bending of the fiber or even variations in temperature along the fiber length, then the speed of one polarization component will increase over the other, making the fiber birefringent. The transmitted pulse gradually spreads and distorts with eventual loss of signal information. Existing fiber lines installed as recently as the 1980's exhibit sufficient birefringence to lose signal content at transmission rates of 10 Gbits/s or more.
While a number of devices have been proposed for compensating PMD in optical fiber, none are completely satisfactory. A conventional PMD compensator comprises a polarization controller and a length of high birefringence fiber (“hi-bi” fiber). The hi-bi fiber is deliberately fabricated to exhibit two orthogonal axes for which aligned polarization components have relatively large differences in speed. The polarization controller receives polarized light from the transmission fiber and orients the polarization so that the slow polarization component is aligned with the high speed axis of the hi-bi fiber. The fast polarization component is aligned with the low speed axis. If the hi-bi fiber is of correct length, the two separated polarization components will reach the end of the hi-bi fiber at the same time, reforming a compact pulse.
A difficulty with this device is that the compensation is fixed by the hi-bi fiber, but the amount of compensation needed varies. The amount of PMD varies with wavelength, temperature and bending of the fiber. Thus needed compensation changes unpredictably with time and cannot be satisfied by a conventional fixed compensation device.
Another proposed PMD compensator uses a tunable, nonlinearly chirped Bragg grating in a length of high-birefringence fiber. The hi-bi fiber provides a different time delay for different polarization components. The nonlinear chirp of the grating provides varying amounts of time delay for different polarization components. The differential polarization time delay can be tuned by tuning the grating. See S. Lee et al., “Adjustable Compensation of Polarization Mode dispersion Using a High-Birefringence Nonlinearly Chirped Fiber Bragg Grating”, II IEEE Photonics Techn. Letters 1277 (October 1999). The difficulty with this approach is that at the same time the nonlinearly chirped Bragg grating compensates PMD, the nonlinear chirp produces chromatic dispersion and higher order PMD. Accordingly, there is a need for tunable compensation of PMD without increasing chromatic dispersion.